Get them into MTBing and leave the youngest at home from time to time?
I'm fortunate that here in Long Island there are plenty of easy trail systems that let the kids get their feet under them. As a result, when my youngest was 3 and a half, and the other two wanted to join me on the trails, I hooked the trail-a-bike to the mtb and hit the mountain bike trails. I could move at a decent pace, really just got a better workout, but it made it easier to go slower for the older kids to keep up. just pay good attention to handlebar clearance. It is fun when they share the sport with you.